Electropneumatic converters are widely used in connection with process control in various applications, for example, where an electric signal is to be utilized to control a pneumatic device such as a positioner, a diaphragm-actuated valves and/or an actuator. The electropneumatic converter is usually located in a control loop between a controller and a positioner or actuator and may be embodied as an assembly in an operated component.
In typical electropneumatic converters, a current signal is applied to a torque motor or a force-coil motor. A flapper, also referred to as a cam or vane, is mechanically coupled to the armature of the torque motor or is attached to the coil in a force-coil motor system. The flapper is located in proximity to the air outlet of a nozzle, known as a baffle-nozzle or flapper nozzle, which is continually pressurized by a source of compressed air. The nozzle normally exhausts to atmosphere. The flapper is moved to the outlet of the nozzle to restrict air flowing through the nozzle and, accordingly, vary the back pressure in the air supply line which feeds the nozzle. The air supply is typically connected to a bellows assembly which moves in response to the back pressure and creates a signal pressure generally in the range of 3 to 15 psig (20.7 to 103 kPa). The output signal pressure is utilized to position or actuate a controlled device.
Known types of force-coil electropneumatic converters utilize various kinds of linkages or counterbalances to operatively connect the coil and vane. Multiple linkages and counter-balances, however, can detrimentally affect both the reliability and accuracy of such converters.